This invention relates to a closed-loop power amplifier distortion controller and to a method of optimising efficiency in a power amplifier.
All power amplifiers introduce distortion to a lesser or greater degree. Distortion may typically be measured in terms of phase and amplitude distortion in which signals passing through the amplifier are subject to undesired phase and amplitude adjustments, i.e. the amplifier has non-linear amplitude and/or phase characteristics. In the case of a power amplifier used for radio frequency transmissions for example for a base station for a cellular mobile radio network, phase and amplitude distortion manifests itself as so-called xe2x80x9cregenerationxe2x80x9d of frequencies outside the desired transmission frequency. Thus in a situation in which a carrier signal, for example, is being amplified, intermodulation distortion and other distortions introduced through non-linearities in the amplifier create side lobes to the carrier which may violate the transmission requirements by producing unacceptable interference in adjacent channels.
For example, in a wideband CDMA (W-CDMA) 3G cellular system, each channel is allocated 5 MHz of spectrum at frequencies around 1.96 GHz. The so-called 3 dB points are spaced 3.84 MHz apart. These requirements are set to allow adjacent 5 MHz channels to be transmitted without unacceptable interference (and indeed the specification is more detailed than this). Thus, if distortion in the power amplifier (or elsewhere in the base station) causes power to be radiated outside the 3.84 MHz band, other communication channels may be adversely affected and the base station will be operating in violation of the official standards and cannot be used.
Several techniques are known in the prior art for xe2x80x9clinearisingxe2x80x9d power amplifiers in order to overcome this problem. The classical approach is to use a feed forward network around the amplifier which tunes the characteristic of the combination of the amplifier and network to compensate for non-linearity in the amplifier. However, these networks are complex to set up and the network surrounding the main power amplifier requires a second xe2x80x9cerror amplifierxe2x80x9d of similar power capability to the main power amplifier. Since the power amplifier makes up a considerable proportion of the cost of a radio transmitter (for example, for use in a cellular radio communications base station), the cost of an additional amplifier for use as an error amplifier is a significant disadvantage.
Another approach described in co-pending U.S. application Ser. No. 09/945,171 entitled xe2x80x9cAmplitude mad Phase Comparator for Microwave Power Amplifierxe2x80x9d is to apply so-called xe2x80x9cpredistortionxe2x80x9d to the signals prior to entering the amplifier. Predistortion is typically the application of an inverse phase and amplitude distortion (which is the inverse of the phase and amplitude distortion which it is known will be applied by the amplifier as the signal passes through the amplifier). Thus by applying the inverse distortion to the signal before amplification, the sum of the predistortion and the inherent non-linearities of the power amplifier result in a significant net reduction in the distortion at the output of the amplifier. However, as described below, further improvements to this technique may be made.
Accordingly it is an object of the present invention to provide improvements to distortion control, power amplifier efficiency and/or maximum allowable transmit power over prior art arrangements and in particular for power amplifiers for use in cellular mobile radio base stations.
In accordance with a first aspect of the invention there is provided a closed-loop power amplifier distortion controller comprising output measuring means operable to measure a predetermined output characteristic of the amplifier output, processing means for processing the measurement produced by the output measuring means, and control means for adjusting an operating parameter of the amplifier responsive to the processed measurement.
The output characteristic may, for example, be phase distortion, amplitude distortion or the frequency spectrum.
In the case at least of phase and or amplitude distortion, it is noted, that this distortion increases dramatically as the clipping point of the amplifier is reached. The clipping point typically is the point at which the output voltage swing approaches the power supply voltages for the amplifier since in typical amplifier designs, the output voltage cannot swing beyond power supply rails. Similarly, clipping may occur when the current supplied by the amplifier exceeds that available from the power supply. Thus phase and amplitude distortion increases dramatically with power output levels at the point that the amplifier clips. If the operating parameter of the amplifier is the power supply voltage and/or the amplifier gain, the gradient of phase and/or distortion error measured at the output of the amplifier against the amplifier power output approaches 1 when clipping occurs. Thus as described below, by adjusting the operating parameter of the amplifier to cause the amplifier to operate just below clipping (as determined by measuring the predetermined output characteristic of the amplifier) the amplifier may be operated at maximum power output (for a given gain and/or power supply voltage) which improves the efficiency.
Similarly, if the output characteristic is the frequency spectrum, the power supply voltage and/or amplifier gain may be varied until the output frequency characteristic is just within an acceptable margin of a desired frequency mask (as discussed in the introduction above). These two approaches typically produce the same result since the frequency mask will be grossly violated (due to phase and amplitude distortion) when the amplifier reaches the clip point and thus operating the amplifier to remain within an acceptable margin of the frequency mask will typically operate the amplifier just at or below its clip point.
In accordance with a second aspect of the invention therefore the invention provides a method of optimising efficiency in a power amplifier comprising measuring an output characteristic of the amplifier which is representative of distortion introduced by the amplifier, and controlling an operational parameter of the amplifier responsive to the measured characteristic.
Using this method to operate the amplifier at or just below its clip point improves efficiency as described below because amplifier efficiency is generally improved when the amplifier is being xe2x80x9cdrivenxe2x80x9d hard. Thus typical efficiencies of 18-20% may be achieved by driving the amplifier at or close to its clip point whereas present constructions in which an operating margin must be built into the circuit design to allow for component variations and temperature variations, are only able to achieve amplifier efficiencies of the order of 10%.
As mentioned above, power amplifiers make up a significant proportion cost of a cellular mobile base station. Thus an increase in efficiency means that potentially lower numbers of power amplifiers are required for a given transmitted power, and/or reduced capacity power supplies and cooling plant may be required. All of these reductions are desirable in terms of cost of base stations, and environmental aspects such as the size of the base station and overall electrical efficiency of the base station.
Other aspects and features of the present invention will become apparent to those ordinarily skilled in the art upon review of the following description of specific embodiments of the invention in conjunction with the accompanying figures.